Recent experimental and theoretical studies provide solid evidence for a sliding microtubule mechanism for flagellar movement, in which bending is the result of sliding generated between tubules by an ATP driven mechanochemical cycle involving the dynein arms. Work is now focused on the details of the mechanochemical cycle, which may be similar to that occurring in muscle, and on the control mechanisms responsible for oscillation and particular patterns of propagated bending waves. Detailed understanding of these mechanisms in simple flagella may aid our understanding of other microtubular systems which generate movement, of the active sliding process in muscle, and of the mechanism of movement of the more complex tails of mammalian spermatozoa. The flagellum forming the tail of simple spermatozoa such as sea urchin spermatozoa is particularly suitable for experimental studies. These sperm flagella generate regular, planar bending waves which can be readily and precisely photographed. Normal movements can be reactivated by ATP following demembranation with Triton X-100, and large quantities can be obtained for biochemical measurements. Our work will mainly focus on measurements of both movement parameters and rates of ATP dephosphorylation of demembranated flagella subjected to a variety of experimental probes. The quantitative data from these experiments is needed for comparison with predictions of theoretical models which are being developed concurrently.